The present invention relates to a process for detecting totally or partially hidden non-homogeneities (faults, such as cracks, bubbles and the like) in opaque materials, by means of microwave radiation.
Cameras are frequently used to examine inaccessible cavities such as sewage pipes for example. In this manner, the structures in the interior, such as the state of the interior wall of the pipe can be examined. In addition to the surface condition of the interior wall of the cavity, however, the adjacent surroundings may also be of interest, and in such instances, inspection by camera must be supplemented with examination by other sensors, which may be based, for example, on the reflection of microwave radiation in the surrounding medium. Therefore, special radar systems have been developed in recent years to inspect the surroundings of bore holes, including pipes from the interior.
According to the state of the art (DASA--company brochure "GeoTelKanal Tiefenradar fur die Kanalinspektion" (October 1992); M. Bockmair, A: Fisch, K. Peter: "Georadar --Erkennung von Schaden und Vortriebhindernissen" ("Suchen Sehen Senieren--Internationales Symposium fur Wasser/Abwasser in Lilndeu Mar. 11--13, 1993); company brochure Riooltechnieken Nederland, Schwieweg 60, 2677 AN Delft (November 1990)), these sensors consist of a pair of directional antennas (i.e., a transmitting and a receiving antenna) which are disposed side by side, and which preferably transmit and receive microwave radiation along a principal direction. These antennas are designed so that the microwave radiation is preferably transmitted or received perpendicular to the axis of the (usually cylindrical) cavity which is being inspected. In known systems, the transmitter and the receiver have hitherto been disposed along the axis of a carrier vehicle, with the axes of their directional characteristics (that is, the axes of their respective transmission or reception patterns) aligned approximately parallel to each other and perpendicular to the carrier axis (and hence, perpendicular also to the axis of the cavity). For inspection of bore holes, sensors of this type can be entered into the bore in place of a drill head. By turning the sensor about the axis of the bore hole, complete coverage of the surrounding medium can be performed.
In systems of this type, however, two different problems occur:
First, due to the alignment of the transmitting antennas approximately perpendicular to the wall of the cavity, strong reflections frequently occur at the inner and outer surfaces of the cavity wall, which reflections reach the receiving antennas directly. The intensity of the overall received radiation is highly dependent on the angle of the wall and the objects located behind it in relation to the antennas. Due to the frequency used, this dependency on the angle is much stronger than the dependency on the material or on its complex refraction index. In particular, in inspecting pipes, which usually have a moisture film on the pipe surface, the strength of the average received signal depends primarily on the geometry and alignment of the surface of the wall relative to the antennas.
Detection of the average received intensity therefore permits no conclusion concerning the state of the pipe or of the surrounding material, which must be evaluated by indirect or direct time resolution, frequency resolution or phase resolution of the received radiation by exploiting the propagation velocity of the microwave radiation (e.g. by means of frequency modulation techniques or pulse radar). However, due to the high propagation velocity, very complex, fast (and therefore very expensive) electronics are essential for inspecting the surroundings, because such components can largely suppress the signals from the interface layers (interior/pipe surface and pipe surface/homogeneous pipe surroundings), which are frequently very strong compared to the reflection that are of interest with respect to the pipe walls and surroundings.
Second, when systems of this type are used alone, the walls and/or the medium located behind it are preferably inspected in only one direction. Thus, relatively large areas of the surroundings cannot be covered without additional measures. Complete inspection, therefore, has hitherto only been realized in the case of bore holes, in which a radar sensor can be rotated instead of a drill. In this case, total coverage of the surrounding earth is possible by rotation of a pair of transmitter-receiver antennas. In prior art systems (such as, for example, pipe robots) in which the carrier vehicle does not rotate, a rotatable holding means for the antenna has to be provided if complete inspection of the surroundings is to be carried out.
The object of the present invention is to provide an inspection process of the generic type described hereinabove, which achieves improved local resolution of totally or partially hidden non-homogeneities in opaque media, by means of microwave radiation.
This and other objects and advantages are accomplished by the inspection process according to the invention, in which the antennas are aligned in such a manner that most of the radiation reflected at the (smooth) pipe wall, and at the interface between the pipe wall and the homogeneous earth, does not reach the receiving antennas as shown, for example in FIG. 1. This is achieved by inclining at least one antenna so that its average directional characteristics is at an oblique angle relative to the axis 20A of the carrier (carrier vehicle ) 20 (which usually corresponds to the axis of the cavity). Hence, the microwave radiation impinges on the surface of the medium obliquely. If only one combined transmitter-receiver antenna is utilized, its directional characteristic axis must be oblique to the carrier axis 20A and therefore to the surface to be inspected (deviating from the perpendicular line by more than 5 degrees).
One advantageous embodiment of the invention utilizes a system of one transmitting antenna and a multiplicity of receiving antennas, so that the signals generated from the one transmitting antenna inside the pipe (and the surrounding medium) can be received by at least two antennas. A technically equivalent system may be composed of a multiplicity of transmitting antennas and one or a multiplicity of receiving antennas, which are not disposed one to one with the transmitting antennas. In this embodiment, the signals are generated in succession by the transmitting antennas, and are measured by the receiving antenna(s). From the different measured signals, the position of a fault is detected using known transmitting and receiving characteristics, based on the direction of the angle region defined by the directional characteristic axes of the antennas operated in the same manner. If the signals are indirectly or directly time, frequency or phase resolved, the angle position of several differently spaced faults can be detected.
With an alignment of the antenna relative to the wall of a pipe and the surrounding homogeneous earth such as described above (that is, using a single directional microwave transmitting-receiving antenna, or a multiplicity of directional microwave antennas), the radiation is reflected by the respective air/pipe and pipe/surrounding earth interfaces, back to the combined transmitting-receiving antenna or the receiving antenna, only to a small extent. Only a fault, such as a crack in the wall of the pipe, or air chamber or moisture dome in the vicinity of the pipe, scatters the radiation back to the receiver. Thus, the existence of a fault in the surrounding medium can be determined from the average received intensity, and the ratio of the "interference signal" (radiation from the wall of the medium which impinges on the receiving antenna) to the signal which is of interest (radiation reflected or scattered at faults in the pipe or the surroundings onto the receiving antenna) is considerably diminished.
In prior art antenna systems, depending on the structure of the pipe surface and its alignment in relation to the antenna, this "interference signal" can be orders of magnitude stronger than the signals from the non-homogeneities to be detected, and accordingly can be suppressed only directly or indirectly via running-time dependent effects. For this reason, it is practically impossible to detect non-homogeneities inside pipes, (that is, between the exterior and interior walls of the pipe) with the known radar processes.
Furthermore, in prior art antenna systems, the signals received from faults in the medium are also more dependent on its surface structure so that interpretation of the measured reflections is very complicated, and usually not clear. Here too, the dependency of the detected intensity on the angle of the surface relative to the antennas can be greatly reduced by means of the invented measurement of the intensity of the back scattered waves, permitting a considerably simplified and clearer interpretation of the measurements.
If manual rotation of a transmitting and receiving antenna pair is undesirable or impossible, the angular disposition of a detected dot-shaped fault relative to the antennas can be determined only by means of an arrangement of a multiplicity of transmitting and receiving antennas according to the invention, for example, in belts placed around the circumference of the carrier vehicle, which are rotated in relation to each other about the axis of the vehicle. For extensive faults, however, an average angular position (weighed with the directional characteristic of the transmitting-receiving antennas) can be detected.
In a further embodiment of the present invention, the antennas are composed of flat or curved portions. By this means in contrast to the known systems, which require a special carrier vehicle, microwave sensors or radar sensors can be set up which require very little space, and can be placed on a conventional camera inspection vehicle.
If harmonic waves of the base frequency can be emitted or received with these or other types of antennas, excited or detected sequentially by both the fundamental wave or a multiplicity of harmonic waves, the resolution of the spatial position of faults can be improved using the same system of transmitting and receiving antennas, by taking into consideration the frequency-dependent varying penetration depths of the microwave radiation and its varying transmission and reception characteristics.
Furthermore, utilizing the strength of the back scattered microwave radiation, which is dependent on the complex refraction index of the non-homogeneity at the particular frequency, the type of fault (cavity, stone, water bubbles etc.) can be accurately identified.
Moreover, even in the simplest application, in which the individual transmitting antennas are excited in succession by means of unmodulated or amplitude-modulated microwave radiation, it is possible to determine approximately, based on the different depth of penetration of the microwave radiation, the position of a fault in respect of the average radiation direction.
For total coverage of the surroundings about a given carrier vehicle, a multiplicity of transmitting and receiving antenna pairs can be disposed on the vehicle. The number of pairs of antennas is usually selected such that multiplication of the aperture angle of the transmitting or receiving characteristic by the number of antenna pairs, yields a value larger than 360.degree.. In order to permit the simplest and best screening of the transmitter and receiver, according to the state of the art, if possible, both the transmitting antennas and the receiving antennas are disposed in the shape of a polygon, in which the radiating and receiving directions are approximately perpendicular to the axis of the carrier vehicle and its projections are parallel in pairs in a plane perpendicular to this axis, i.e. located on the carrier vehicle are one transmitting and one receiving belt respectively. Thus, complete coverage of the surrounding medium is possible; however, the angular position of detected faults in the plane perpendicular to the axis of the belt can only be accurately determined on a fraction of 360.degree./n.
On the other hand, much more accurate determination of the angular position compared to the state of the art is possible if, according to the invention, transmitting and receiving belts are turned in relation to each other in such a manner that radiation emitted by a transmitting antenna and scattered or reflected at the faults can be detected with at least two antennas. With the known transmitting and receiving characteristics, subsequently the angular position of the faults can be determined from the difference in the detected intensities and phases. As a rule, the same information can be determined if two transmitters emit microwave radiation in succession and the radiation scattered or reflected at the fault is detected by at least one antenna.
By placing more antenna belts on the carrier vehicle, the resolution of the spatial position of the can be further improved.
The aforedescribed processes are independent of the type of excitation and detection of the microwave radiation. The advantages are readily exploitable with very simple sensors, with CW (constant wave, with constant amplitude) or AM (amplitude modulated) excitation of an antenna or a multiplicity of antennas. With such sensors, however, a resolution in the radiation direction can only occur roughly over the overlapping regions of the transmitting and receiving characteristics or the varying range of the microwave radiation at varying frequency. If, however, additional radar techniques are utilized for excitation and temporal resolution of the received radiation, the distance of a multiplicity of faults located one behind the other in the radiation direction to the antennas can also be determined.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.